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lagoons (22 %) and not in the main river, probably because these lagoons do not completely
dry-out during the low water season. On the other hand, the sex-ratio (45.8 % males and 54.2
% females) of these captured animals, did not significantly differ from a 1:1 sex-ratio. Many
females were with calves, and there was no segregation of sexes as was observed by Martin &
da Silva (2004b). Although our collections occurred during low-falling water season, the
same time of year when Martin & da Silva observed the low level of segregation of sexes in
Mamirauá. Nonetheless, we did not observe sexual segregation in diverse lagoons of the
Ucayali and Marañón rivers in the Peruvian Amazon during high rising-high water for this
species, which could be in disagreement with that postulated by Martin & da Silva (2004b).
Although, I have no evidence that the sexual segregation was present in the Peruvian rivers
analyzed, this does not necessarily affect the social reproductive system obtained by the
Chesser's simulations because the reproductive social system is really important in low water
areas where males and females search for each other to breed if there is fidelity for the
reproduction place. However, such as I commented before, in areas of extreme density or
with enormous population size, such as Mamirauá (Martin & da Silva, 2004a, estimated about
13,000 pink river dolphins in the 11,240 km
2
of the Mamirauá reserve), specific genetic
analyses must be developed to determine if the genetic social reproductive parameters are
similar to those estimated in the major part of
Inia's
geographic distribution, where
population sizes are not as large (such as I presented here), or are clearly different.
Additionally, all the areas where observational and genetics results were obtained correspond
to white-water rivers (that is, rivers that drain the Andes mountains; Meade & Koehnken,
1991). It would be of interest to carry out micro-genetic structure analyses of pink river
dolphins from clear-water rivers (draining, for instance, the Colombian and Venezuelan
Llanos, like the Cinaruco River, or rivers from the upland Guyana Shield, such as the
Ventuari River) and acid black-waters (as the Negro River in the Brazilian Amazon or the
Atabapo River in Venezuela; Meade & Koehnken, 1991) to know if these diverse ecological
systems have similar social reproductive parameters that I determined for the pink river
dolphin population from a main white-water affluent (Napo/Curaray River) of the upper
Amazon River. It seems that pink river dolphin populations in black-water rivers are scarce
compared to those in white-water rivers. For instance, Pilleri & Pilleri (1982) affirmed that
the density of
Inia
was 50 times less in black-water systems such as in the upper Orinoco and
the Casiquiari channel compared to a white-water river like the Apure River. In fact, some
black water tributaries, for example, of the Napo River in Ecuador do not contain
Inia
populations (Tagliavini & Pilleri, 1984). It would be of additional interest, and decisive for
conservation programs, to determine how anthropogenic activities including fishing with
diverse types of nets (da Silva & Best, 1996), use of motorized boats and colonization of river
banks alter the micro-genetic structure of these amazing dolphins.
A
CKNOWLEDGMENTS
Economic resources to carry out this study were obtained from Colciencias (Grant 1203-
09-11239; Geographical population structure and genetic diversity of two river dolphin
species,
Inia boliviensis
and
Inia geoffrensis
, using molecular markers) and the Fondo para la
Accion Ambiental (US-Aid) (120108-E0102141; Structure and Genetic Conservation of river
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